why not just use AsRock H81 Pro BTC? built for mining. I have it, and it works without problems under ubuntu 13.10
For Intel there are a few. For amd not so many.
Do you have x1 risers connected into your x16 slots to run 6?
More to the point, is it x1 usb risers or x16 to x16 ribbons?
before I bought the H81, I also wanted to go with a AMD cpu. However, the price of the H81 is perfect, it works good and offers a lot. The whole rig (celeron cpu & 5 x 750Ti) only pulls 240Watt from the wall (according to my kill-a-watt), so maybe you want to think about going with AMD again. If you build 20 rigs, you can save A LOT of money (just by going with the cheap H81)!
Cpu farm, I believe Intel is the way, just on power saving.
I have seen 7 cards being run on a msi gd65 motherboard.
The other Intel one is Haswell only, pretty cheap to set up, low power overheads. And will support 6 gpus and may allow for you to use the on aboard so all gpus run at full power
I have seriously considered that but I read from reddit that alot of people having problem with this board under Linux to get the 6 cards working. Have you tried the 6. card in your board under ubuntu 13.10, bigjme?